Steel pipes such as oil country tubular goods used for the excavation of underground resources are normally connected with each other by threaded joints. A threaded joint for steel pipes generally has a pin-box structure comprising a pin having male (external) threads and a box having female (internal) threads which threadingly engage with the male threads. Typically, a pin is formed on the outer surface of both ends of a steel pipe, and a box is formed on the inner surface of a separate connecting member referred to as a coupling. This type of threaded joint is referred to as a coupling-type threaded joint. There are also integral threaded joints in which a pin is provided on the outer surface of one end of a steel pipe and a box is provided on the inner surface of the other end of the pipe, and steel pipes are directly connected to each other without using a coupling. The threads formed on the pin and the box of a threaded joint are typically selected from trapezoidal threads (buttress threads), triangular threads (round threads), hook threads, and the like.
One type of threaded joint for steel pipes is a so-called API joint, which is a threaded joint which has been standardized by the American Petroleum Institute. FIG. 4 is a cross-sectional view showing the longitudinal cross-sectional shape of a coupling-type API round threaded joint.
A coupling-type round threaded joint 1 shown in FIG. 4 comprises pins which are formed on the outer surfaces of the ends of steel pipes 2 and 3 and boxes which are formed on the inner surface of a short connecting member (a coupling) 4. The pins are formed by providing tapered male threads 2a and 3a in the shape of round threads having a generally triangular cross-sectional shape in the axial direction of the pipe on the outer surfaces of the ends of the steel pipes 2 and 3. The boxes which oppose the pins are formed by providing tapered female threads 4a in the shape of round threads which threadingly engage with the male threads 2a and 3a of the pins on the inner surfaces of the coupling 4 at both ends thereof. In addition to the female threads 4a, each box has an unthreaded sleeve 4b having an inner diameter which is larger than the outer diameter of the pipes 2, 3 at both lengthwise ends of the coupling 4. The unthreaded sleeve 4b serves as an insert guide.
Makeup (tightening) of a threaded joint is carried out by mechanical tightening using a tightening mechanism which is referred to as power tongs and which is operated by oil pressure or water pressure. Makeup is completed when a prescribed torque reaction force is reached or when a predetermined makeup position is reached. When makeup of a pin and a box is completed, the threads of the pin and box are in the state of interference fitting in which the threads are tightened beyond the position in which they are just contacting, so an extremely strong connection is achieved.
Galling of the threads of a threaded joint develops due to slight problems at the time of makeup such as misalignment of the axes of the male threads and the female threads, the presence of sand or rocks on the threads, or an insufficient amount of lubricant. Normally, a tightening mechanism for threaded joints, lubricant for the threads, and the tightening procedure in the field are controlled so that galling does not take place. However, it is sometimes not possible to adequately carry out such control at oil wells in severe environments which have become increasingly common in recent years. Therefore, the problem of galling often takes place.
There have been many proposals which attempt to prevent galling (or improve galling resistance) by improving a lubricant which is applied to the surface of a threaded joint, by improving a lubricating coating which is sometimes framed on the surface, or by improving a method for surface treatment performed on the surface (such as surface roughening treatment). It has also been proposed to prevent galling by modifying the thread shape.
Threaded joints for steel pipes in which prevention of galling is achieved by improving a lubricating coating or surface treatment often require care in handling so as not to damage the lubricating coating or surface treatment. During transport of pipes to an oil well or due to rough handling in the field or careless control during storage, defects such as peeling of the lubricating coating can easily take place. If such defects occur, it is not possible to improve galling resistance.
Concerning improvements in thread shape, U.S. Pat. No. 6,447,025 discloses a threaded joint for steel pipes in which the rounding of the crests of the male threads of a pin with round threads is made larger on the stabbing flank side than on the load flank side. As a result, scratches and abrasions at the time of thread makeup can be prevented. However, although such a thread shape can prevent damage to the flanks of the opposing threads of the pin and the box at the start of makeup, it cannot prevent galling caused by a local high contact state as discussed below.
In the threaded joint for steel pipes disclosed in JP 2001-56075 A, the pitch of the male threads of a pin is made 1-7 micrometers smaller than the pitch of the female threads of a box. As a result, an increase in the contact force on the thread flanks caused by elongation of the pin in the axial direction due to the Poisson effect in the state of interference fitting is suppressed and galling resistance is increased. Namely, this threaded joint for steel pipes can exhibit the effect of increasing galling resistance in a state in which the threads of the pin and the box are adequately tightened to achieve interference fitting so that the pin has elongated in the axial direction due to the Poisson effect. However, in the initial stage of makeup in which the threads of the pin and the box are not yet interfering, due to the difference of the thread pitch between the pin and box, an unnecessary contact force develops on the thread flanks, and galling can easily develop.